1. Field of the Invention
The present invention relates to a fluorine-containing copolymer which improves extrusion moldability for coating an electric cable with an insulating resin and which is capable of suppressing the occurrence of molding faults over long term operation even when coating at high speed.
2. Description of the Related Art
Tetrafluoroethylene (TFE)/hexafluoropropylene (HFP) copolymer has superior heat resistance, chemical resistance, extrusion moldability and the like, and in addition, has superior electric insulating property and high-frequency property with a low dielectric tangent. Therefore, it is used for insulating cable such as a cable and a wire, and such insulated cable is suitably used as a communication cable. The communication cable includes a data transmission cable such as a LAN cable.
TFE/HFP copolymer also has low flammability and low smoking properties. Thus, insulated cable made from such a copolymer can be used as a plenum cable, which is laid, for example, on the back of a ceiling of a building (plenum area) and strictly regulated for preventing the spread of fire.
The insulated cable comprises a core wire such as a cable and an insulating material formed from a resin such as a TFE/HFP copolymer coating the core wire. In general, the insulated cable is manufactured by extrusion coating in which molten resin is extruded in the shape of a tube, drawn down by inserting a core wire through the center portion of the resin tube in its axial direction, and the core wire coated with the resin is then taken up.
The term xe2x80x9cdraw-downxe2x80x9d as used herein means extending a molten resin extruded from a die having an opening of relatively large sectional area to its final intended dimensions. The draw-down is characterized by a draw-down ratio (DDR), which is the ratio of the sectional area of the opening of the die to the sectional area of the insulated material of the final product. In general, the draw-down ratio is suitably from 50 to 150.
The term xe2x80x9cinsulatedxe2x80x9d cable as used herein means a cable or wire coated with an insulating material.
Preferably, the draw-down operation is carried out in so that the outer side and inner side of the molten resin, extruded from the die in the form of a tube, are evenly drawn down. This evenness is expressed as draw ratio balance (DRB).
Since in a conventional draw-down operation the resin is drawn as described above, a cone-break sometimes occurs during an extremely short period between the time the resin is extruded from an opening of the die until it contacts a core material. The cone-break is the largest cause of loss in productivity. This is because cone breaks, when they occur, requires restarting the extrusion process and machine utilization is decreased.
In recent years, an increase in molding rate has been desired to enhance productivity and to reduce cost, and there is a demand to increase the cable coating rate by increasing the winding rate of the insulated core wires. The trend of coating the cable at high speed generally increases the occurrence of cone-breaks even though the draw-down ratio is similar.
A technique for reducing the occurrence of cone-breaks is proposed in Publication No. WO 00/44797, in which a TFE/HFP copolymer having a melt flow rate (MFR) of 24 (g/10 minutes) is used and the coating rate of applying an insulating resin to the cable is lower than 2000 ft/minute. Indeed, this copolymer is effective at a coating rate of lower than 2000 ft/minute. However, this technique does not satisfactorily reduce the occurrence of cone-breaks at faster coating rates.
In the coating extrusion, the thickness of the insulating material of the insulated cable is determined by the desired electrical characteristic, and the size of the opening of the die is determined by selecting a draw-down ratio and draw ratio balance suitable for the structure of the insulated cable.
In the case of increasing the cable coating rate without changing the size of the opening of the die, the problem of developing a rough skin on the surface of the resin passing through the opening generally occurs when the velocity of the molten resin in the die exceeds a certain velocity limit. This rough skin is referred to as melt fracture, and it develops when the velocity of the resin exceeds the critical shear rate during melt fluidizing.
The trend of coating cable at high speed also involves the problem of exacerbating the extent of the melt fracture. Excessive melt fracture sometimes leads to a situation where the insulating material does not completely cover the core material, and causes an increase in molding faults such as spark-out and cone-breaks.
As one technique for delaying the occurrence of melt fracture and enabling high-speed coating of about 1.5 times faster than conventional rates, U.S. Pat. No. 5,703,185 discloses that vinyl ether for use as a third monomer component of a TFE/HFP copolymer is changed from perfluoro (propyl vinyl ether) to perfluoro (ethyl vinyl ether). However, this technique encounters difficulty in reducing molding faults such as cone-breaks and the like at high-speed coating rates.
It is therefore an object of the present invention to provide a fluorine-containing copolymer which improves extrusion moldability for coating an electric cable with an insulating resin, and which is capable of considerably reducing molding faults such as spark-out and cone-breaks over long term operation even when coating at high speed.
The above object of the present invention is achieved by providing a fluorine-containing copolymer obtained by copolymerizing tetrafluoroethylene, hexafluoropropylene and optionally perfluoro vinyl ether as component monomers,
wherein a weight ratio of tetrafluoroethylene, hexafluoropropylene and perfluoro vinyl ether units constituting said fluorine-containing copolymer is 70 to 95:5 to 20:0 to 10, respectively;
the fluorine-containing copolymer having:
a melt flow rate 30 (g/10 minutes) or more:
a volatile content index of 0.2 weight % or less; and
a stress relaxation modulus G(t) (unit: dyn/cm2) which satisfies the following formula at t=0.1 second when measured at a temperature of 310xc2x0 C.
G(0.1) greater than 7xc3x97106xc3x97Xxe2x88x921.6143xe2x88x923000 
where X denotes the melt flow rate in units of g/10 minutes.
Hereinafter, the present invention will be described in greater detail.